Drop2 plugin by elsewhere

Couldn't find any thread for that plugin so i had to open one myself.

The plugin has an extensive list of chord voicings and variations, kudos for elsewhere for taking the time to write down all that.

I took the liberty to edit the code and fixed some issues and limitations as follows:

• plugin now keeps slurs and any other ornaments/articulations intact
• triplets now expand correctly [although the old "wrong" way of expanding triplets sometimes gave an interesting and unexpected result :) ]
• plugin modifications are now restricted to selection instead of operating on the whole staff.
• Added MU4 support

I also changed the way it expanded the bass notes in the bass staff: all notes including bass notes now expand in the same staff as the melody note. (don't need 2 staves to expand drop 2 anymore). I found that to be more useful for when arranging and would later want to explode chords to instruments. In case the bass notes are needed in the bass clef, just use "filter note levels" plugin for an easy and quick copy/paste of bass notes to bass clef.

It would be useful to adjust the plugin for drop 3 as well, but by the way the chords arrays were originally written i think it might need some work to edit everything.